1. Field of the Invention
The present invention relates to a method for the removal of deposition on an optical element, a method for the protection of an optical element, a device manufacturing method, an apparatus including an optical element, and a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that example, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g. of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
The source of EUV radiation is typically a plasma source, for example a laser-produced plasma or a discharge source. A common feature of any plasma source is the inherent production of fast ions and atoms, which are expelled from the plasma in all directions. These particles can be damaging to the collector and condenser mirrors which are generally multilayer mirrors, with fragile surfaces. This surface is gradually degraded due to the impact, or sputtering, of the particles expelled from the plasma and the lifetime of the mirrors is thus decreased. The sputtering effect is particularly problematic for the collector mirror. The purpose of this mirror is to collect radiation which is emitted in all directions by the plasma source and direct it towards other mirrors in the illumination system. The collector mirror is positioned very close to, and in line-of-sight with, the plasma source and therefore receives a large flux of fast particles from the plasma. Other mirrors in the system are generally damaged to a lesser degree by sputtering of particles expelled from the plasma since they may be shielded to some extent.
In order to prevent the damage of the collector mirror by debris particles, U.S. Patent Application Publication 2002/0051124 A1 discloses a cap layer on the mirror surface to protect the mirror from sputtering damage caused by fast ions and atoms expelled from a plasma source. Hydrocarbons are added to a space containing the mirror, and they physically or chemically adsorb to the surface of the mirror and thus form a protective layer on the surface. This surface layer is made up of the hydrocarbon molecules and possibly other contaminant particles present in the system as impurities, together with any further molecules which are introduced into the system from the gas supply. When the fast ions and atoms produced by the plasma hit the surface of the mirror, they contact the protective layer thereby dislodging the hydrocarbon molecules from the cap layer, and damage to the mirror surface itself is avoided. A dynamic cap layer may be used. This is a cap layer which is continually sputtered away and replaced with further molecules and thus the thickness of the layer remains substantially constant or within an acceptable range. In order to achieve this, the reflectivity of the mirror and/or the background pressure of the space are monitored.
In the near future, extreme ultraviolet (EUV) sources may use tin or another metal vapor to produce EUV radiation. This tin may leak into the lithographic apparatus, and will be deposited on mirrors in the lithographic apparatus, e.g., the mirrors of the radiation collector. The mirrors of such a radiation collector are foreseen to be multilayered and may have a EUV reflecting top layer of ruthenium (Ru). Deposited layers of more than approximately 10 nm tin (Sn) on the reflecting Ru layer will reflect EUV radiation in the same way as bulk Sn. It is envisaged that a layer of 10 nm Sn is deposited very quickly near a Sn-based EUV source. The overall transmission of the collector will decrease significantly, since the reflection coefficient of tin is much lower than the reflection coefficient of ruthenium. The method of U.S. Patent Application Publication 2002/0051124 A1 is not suited to remove, for example, Sn deposition from the surface of optical elements like mirror surfaces, nor is it suited to remove, for example, Si deposition from optical elements. U.S. Patent Application Publication 2002/0051124 A1 also does not address sputtering of particles and other contaminants on optical elements. An improved method is therefore desireable to address this problem.